1. Field of the Invention
The present invention relates to test apparatus for Electromagnetic Compatibility (EMC) testing of electronic equipment, and more particularly to the use of a stripline antenna having a width of less than half the wavelength of the highest frequency of the electromagnetic radiation, suspended within an enclosure into which is placed the electronic equipment to be tested.
2. Description of the Prior Art
The constraints being put on the manufacturers of electronic equipment with regard to the susceptibility of electronic equipment to electromagnetic radiation and the levels of electromagnetic radiation which are allowed to be emitted from electronic equipment have recently become more stringent. Particularly in Europe, a directive is due to come into force which requires all manufacturers of electronic equipment to certify that their products neither emit RF energy nor are susceptible to it, within given limits. The proliferation of electronic equipment e.g. mobile telephones, facsimile machines, video recorders, etc. has made the subject of Electromagnetic Compatibility (EMC) more relevant for the commercial and industrial world. The military have been involved in this field for a very long time and most of the test methods and failure criticism are derived from their experience.
In view of the more stringent requirements for EMC, many manufacturers have decided to carry out the necessary tests themselves rather than relying on specialist consultants for example.
There is therefore a need for inexpensive and simple EMC test equipment.
The European Standard which has been ratified by CENELEC, which is a directive which has been brought into force within Europe, an arrangement for measuring the susceptibility of electronic equipment to electromagnetic radiation is disclosed and stated as being the standard arrangement which is to be used for EMC testing. The test arrangement disclosed in the directive is shown schematically in FIG. 1 and the specific proposed configuration is shown in FIG. 2.
In order to test electronic equipment the aim is to generate a homogeneous electromagnetic wave under free space conditions. This can be simulated by a guided wave of the TEM (Tranverse Electromagnetic) mode travelling between two flat conducting plates. In this case the electric field component is perpendicular, and the magnetic field component parallel to the conductors.
As can be seen generally in FIG. 1 with the more detailed constructional details given in FIG. 2, the stripline TEM device comprises two spaced parallel metal plates 1 and 2. The RF field is applied by an RF generator 6 across the plates 1 and 2 and a terminating impedance 7 is provided to complete the circuit.
FIG. 2 shows the suggested constructional details wherein the two plates 1 and 2 are spaced by plastic bracings 3. A non-metallic support 4 is provided on to which the electronic equipment 5 undergoing tests can be placed.
It is stated in the standard that this construction is usable up to 150 MHz. This is severely limiting since it is necessary for some electrical equipment to be tested at higher frequencies.
One commercial form of the TEM cell is the range of EMCO TEM cells available from Electro-Metrics Limited of Shefford in the United Kingdom. The range of TEM cells can provide for EMC testing in the ranges DC to 100 MHz to DC to 500 MHz depending on the size of the TEM cell. The EMCO Model 5101 TEM cell is the model which is capable of operating in the frequency range DC to 500 MHz. The dimensions of this cell are 30.48 cm sq. Thus although the frequency range available for EMC testing is as high as 500 MHz, the size of electronic equipment which can be tested is extremely limited. This is an inherent limitation of the TEM cell design. In order to provide the required simulated homogeneous electromagnetic waves under free space conditions within the parallel plate design of the TEM cell for high frequencies it is necessary to reduce the dimensions of the plate. It is not possible to construct such a TEM cell of useful capacity for a higher frequency operation.
For EMC testing at higher frequencies well into the GHz range, Electro-Metrics Limited have developed the GTEM! (Trade Mark) cell. Three different sizes of GTEM! are available, the smallest of which is illustrated in FIG. 3 and has a length of some 3 meters, a width of 1.6 meters and and height of 1.7 meters but provides only a useful internal height of 0.5 meters.
As can be seen in FIG. 3, the GTEM! comprises a flared enclosure which tapers out from the input end 10. This arrangement comprises a tapered stripline 11 the impedance of which is carefully matched. At the far end of the cell there is provided RF absorptive material 12 to terminate propagating waves. The small taper of the cell propagates an undistorted evenly distributed TEM wave without propagating higher order modes. Access to the interior of the cell is provided in the form of a door 13 and electrical feeds through the cell walls are provided by an electrical distribution box 14.
The GTEM! can operate up to 20 GHz but is expensive and is very large.